The construction, maintenance, and habitation of buildings is the single biggest contributor of greenhouse gases, and heating and cooling systems account for up to 50% of energy used in buildings in the United States. Reducing the energy used to heat and cool buildings is critical to energy conservation, and energy conservation is critical to both national security and economic prosperity.
A variety of governmental and non-governmental organizations are causing a tightening of energy efficiency standards as they pertain to buildings. In 1992, Congress passed the Energy Policy Act which required states to have building codes that set efficiency standards that are at least as stringent as the Federal standard. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standard 90.1 “Energy Standards for Buildings Except Low Rise Residential Buildings” is followed by most states who update their state codes as 90.1 is updated. The 2030 Challenge, adopted by the US Conference of Mayors, AIA, USGBC, ASHRAE, and other important governmental and non-governmental organizations, requires new buildings to reduce energy use by 60% in 2010, 70% by 2015, 80% by 2020, 90% by 2025, and to achieve carbon neutrality by 2030. Meeting these tighter standards will require improvements in design and construction.
Energy efficiency is not the only demand made of construction. The building envelope, or the enclosure, must withstand occupant loads, wind loads, fire, precipitation, and humidity and condensation, as well as insulate the building. These requirements are codified through a number of standards. In addition to ASHRAE standard 90.1-2007, National Fire Protection Association (NFPA) standard 285 identifies flame propagation requirements for exterior non-load bearing wall assemblies used in non-combustible construction. American Society for Testing and Materials (ASTM) standard E-331 dictates water barrier property requirements. ASTM E-2357 dictates air barrier property requirements. Additionally, exterior-facing and interior, room-facing wall surfaces must be aesthetically appealing.
To meet these different requirements, a building envelope is constructed of a number of different materials, typically applied in layers. The walls of buildings are commonly constructed from frames composed of studs attached at their bases to a wall plate and at their tops to a ceiling plate. A wall assembly is built by attaching multiple building components to and within the frame. Non-combustible walls are commonly constructed with steel framing which provides the basic structure. Layers are applied to the framing to meet aesthetic and performance requirements. These layers typically sheathing, which can be made of exterior grade gypsum board. Gypsum board adds to the strength of the wall, and can provide fire resistance as well as serving as a base for moisture, air, or vapor layers. One or more moisture, air, or vapor resistant barriers formed of material such as asphalt impregnated paper, plastic sheeting, or building wrap is typically located outside the sheathing. Insulating layers such as mineral wool insulation or other insulation reduce heat transfer through the wall. External finishing cladding provides additional protection and makes the wall visually appealing. Drywall, or interior gypsum wall board, is often used to finish an interior wall as well as to provide further fire protection. Fiber batting insulation installed within the cavities between studs contributes reducing heat loss. Walls may also be made of concrete, which also comprises metal support structures, the inclusion of moisture, air, and/or vapor barriers, and fastening systems which hold the various components together.
The studs, or wall frame, provide the structural strength of the wall and form a base to apply the various layers which function to resist wind loads, repel moisture, and maintain internal temperature. The envelope and the methods used to affix its constituent materials to the studs must withstand all forces experienced by the building. Wind blowing against a building exerts a positive pressure on the windward side and suction, or negative pressure, against the leeward side. Depending on the wind, negative pressures can be exerted against other sides of the building or the roof. Internal pressures from stack effect or mechanical systems also act on the building envelope. Thus, all materials need to be attached with fastening systems that can resist not only the weight of the materials themselves, but the compressive and negative forces of wind, and wind-created negative pressures. Fastening systems must also be able to resist deformation. The weight of a facade or exterior cladding installed over several inches of insulation can create a large moment of force and shear force which act on fasteners extending through materials positioned between studs and the exterior cladding. Ineffective fasteners can creep over time, resulting in building component damage and failures. For example, if fastening systems do not protect the interior insulating layers against the forces exerted by exterior layers, insulating materials can be crushed or deformed and lose insulating properties.
For these reasons, building envelopes are conventionally constructed with robust fastening systems such as metal Z gifts installed over the exterior sheathing, allowing insulation to be installed on the exterior and providing a structural base for the cladding. Such fastening systems are strong because they involve a significant amount of material, employ large surface areas which can provide continuous support to components such as panels, and thereby enable the constituent elements of the building envelope to withstand the various forces they are expected to encounter. Unfortunately, cladding fastening systems also tend to undermine the effectiveness of the exterior insulation by creating thermal bridges, commonly lowering the effective R-value of the completed wall assembly to less than required by the energy codes and standards.
Heat flows from higher temperature regions to lower temperature regions through conduction, convection, or radiation. Materials that conduct heat well are called conductors, and materials that do not conduct heat well are called insulators. Thermal resistance is a measure of heat flow. Under uniform conditions, it is the ratio of the temperature difference across an insulator and the heat flux. It is typically expressed as an R-value in construction arts. Conductors have low R-values and insulators have high R-values.
An assembly's effective R-value is calculated by area, by averaging R-values of the various components which are parallel (side by side) and adding R-values of components that are in series (layers). The effective R-value of an assembly will be less than the R-value of the insulation component due to parallel heat flow through more conductive components. In light gauge steel-framed assemblies, heat flows through the steel studs can mean that the R-value of the wall is less than half of the R-value of the insulation used in the wall. Designers, contractors and code officials often mistakenly equate the insulation R value and the wall R value and do not recognize that the metal conducts heat well and significantly reduces the R value of the actual wall below the R value of the insulation itself.
Any thermally conductive part of an assembly that forms a pathway through insulating materials lowers the R-value of a building envelope. Studs and tracks, fasteners, structural members, and cladding support structures are all conductive. When conductive materials are used to fasten materials to wall frames, they contact the frames and extend through insulating layers, creating a path of least thermal resistance from the warm studs to the cold exterior and facilitating parallel heat loss through the wall. The warm building interior will warm wall studs, and that heat is easily conducted to the bracket or Z girt that is in substantial contact with the steel stud, and those brackets and Z girts extend through the insulation so that the heat is channeled to the cold exterior of the building. The surface area of brackets and Z girts that protrude through the insulation especially enables them to radiate the heat into the cold environment outside the insulating layer of the building. This phenomena is also known as a “fin effect.” As a result, the wall in question will have an inadequate R-value and the problem gets worse with increasing levels of insulation. Additionally, the thermal bridges will create warm or cold spots within a wall, which can lead to condensation and resulting mold and water damage.
In response, the conventional approach in the building industry is to use a thicker layer of insulation. However, this solution does not resolve the thermal bridging problem because the cladding is still attached to metal components which extend through the insulation. Adding insulation around thermal bridges has no impact on the conductive nature of the thermal bridge. If enough insulation is added, the wall as a whole may eventually achieve the target R-value, but the problems caused by the creation of warm or cold spots within the wall will persist. The law of diminishing returns applies. Each additional thickness of insulation has decreasing effectiveness. The first few inches of insulation deliver the most efficiency, and each additional inch yields less return. Eventually additional insulation does not contribute any added insulation value. Therefore walls with significant thermal bridges may never meet their intended R value.
A wall with a thermal bridge may be analogized to a bucket with a hole in it. Adding insulation without breaking thermal bridges is like increasing the thickness of the walls of the bucket but not plugging the hole.
Metal fasteners are used despite these disadvantages because of the high strength of metals such as steel, and the fact that metal fasteners can be economically manufactured in a variety of different configurations using known methods. No other material offers this combination of attributes in this context. What is needed are methods of using metal fasteners to fasten elements of a building envelope to studs that interrupt the thermal bridges created by those metal fasteners. However, any such methods must withstand the demands that occupants, environmental forces, and gravity make on building envelopes. The structural integrity of buildings is critical to human health and safety. The fasteners that hold together buildings must withstand the various positive and negative pressures of wind, the substantial weight of the envelope components, and extraordinary events such as hurricanes and earthquakes. Fundamental technologies such as steel brackets connected to steel studs with steel bolts have a long, established history of withstanding these forces and keeping people safe. What is still needed is means of interrupting the thermal bridges created by metal fastening systems which does not impair the structural integrity of those fastening systems.
Conventionally, building envelopes include a structural layer such as gypsum board and an insulating layer such as mineral wool, both of which are affixed to wall frames. However, Dow produces and markets polymeric foam boards which provide both structure and insulation. Dow markets this product under the THERMAX trademark. These polymeric foam boards are described in US Pub. No. 2009/0320397. As Dow notes in the referenced published patent application, however, metal fasteners must extend through the insulating polymeric boards and attach to the metal wall frame. See paragraph 38. These metal fasteners create thermal bridges which compromise the effectiveness of the insulation. Moreover, these boards provide substantial structural strength—far more than mineral wool insulation. Traditional methods of mounting cladding to a conventional wall such as Z-girts do not take advantage of the properties of rigid foam boards, which are capable of withstanding considerable compressive force, especially spread over significant surface area. A method of mounting cladding to foam boards such as THERMAX in a way that does not create thermal bridges would be desirable.